The invention relates to a method and to apparatus for testing a substrate in which a particle beam is directed onto the substrate and emitted secondary particles are detected with a detector and then evaluated.
The substrates to be tested have for example a plurality of microelectronic or micromechanical elements which often have to be checked in an intermediate stage of production. These elements include for example thin-film transistors or other active elements of a liquid crystal display matrix on a glass plate before assembly with the colour filter, connecting networks of a multichip module substrate before equipping it with ICs, transistors of an integrated circuit on a wafer before sawing and bonding, electron emitters and connections of an emitter array for a FED display before assembly with a counterelectrode and evacuation, and sensors of any kind on their particular supporting substrate.
It is common to these elements that they are produced in a plurality of process steps, consisting of masking, etching, depositing etc. In this case faults can occur due to particles, faulty adjustment, defective process equipment and the like. These faults can lead to disruption of the properties of individual elements on the substrate. It is therefore desirable to be able to test the function of each individual element before further processing and to reject the corresponding substrate or to repair it as well as to identify and eliminate shortcomings in the process.
The testing of these elements makes special requirements of the apparatus and the method because of the small dimensions. The mechanical contacting for measurement of electrical quantities increasingly fails because of the size of the measuring points and requires a very high degree of precision. In addition, mechanical contact cannot be tolerated in many cases because of the associated damage to the contact surface. Similar restrictions also apply to the measurement of other quantities, e.g. mechanical quantities, by mechanically acting test devices. Therefore apparatus and methods have been developed which allow contactless functional tests, for example with electron or ion beams. Accordingly electron beam measuring devices and methods for fault analysis of integrated circuits are known.
In the known devices the substrate to be examined is scanned by deflection of a particle beam. However, due to the deflection of the particle beam only a few square centimetres of the substrate are covered. The current multichip module substrates can have dimensions exceeding 20xc3x9720 cm.
Therefore, in order also to be able to test substrates having a large surface area with the aid of a particle beam, a displaceable table has been provided on which the substrate is retained and which can then be moved into a plane perpendicular to the electron beam. In this way the scannable area of substrates can be enlarged to a sufficient extent. However, the drawback has to be accepted that the displacement of the table requires considerably more time than the scanning of the substrate by deflection of a particle beam. Attempts have therefore been made to enlarge the area to be scanned and evaluated on a substrate by deflection of the particle beam.
However, enlargement of the scan area produces the problem that the particle beam must be concentrated to a sufficiently small diameter even in the case of larger deflection angles. In order to achieve this, elements have been provided for dynamic focusing and correction of astigmatism.
During the scanning of larger areas of a substrate by deflection of the electron beam the difficulty also arises of guiding the secondary particles emitted at a specific location to the detector. In EP-B-0 370 276 a special detector and collector construction is described in which the detection characteristic is as uniform as possible over the entire scan area.
The object of the invention is to provide a method and apparatus for improving the testing of substrates having a large surface area.
With ever-increasing deflection angles the detector signal also changes in so far as the location of the secondary particles emitted on the substrate relative to the position of the detector has an increasing influence on the number of secondary particles reaching the detector. In other words, the detector signal varies to a large extent with the location of the emitted secondary particles relative to the position of the detector.
In order to achieve a uniform signal evaluation over the entire area, therefore, the location of the secondary particles emitted on the substrate relative to the position of the detector is taken into account during testing. In this case there are in principle two variants:
1. Means are provided which guide the secondary electrons from a location or site on the substrate to the detector and are controlled in such a way that a detector signal which is independent of the location is set at the detector.
2. The location or site of the emitted secondary electrons is not taken into account until the evaluation, i.e. in a comparison with the desired reference signal, in which case either the detector signals are compared with location-dependent desired signals or the detector signals are corrected as a function of the location and then compared with a desired signal.